Abstract
Purpose :
Genetic variants modulate the population sizes of various retinal cell types in different strains of mice. For example, while there is modest variation in the number of AII amacrine cells between two different inbred strains of mice (A/J and C57BL/6J), their F1 offspring contain significantly more cells than either parental strain, indicating the presence of variants in genes that modulate their number. To identify the genetic source of this variation, we quantified the number of AII amacrine cells in 26 genetically distinct recombinant inbred (RI) strains of the AXB/BXA strain-set, seeking to map variation in cell number to genomic loci. We interrogated genes at those quantitative trait loci (QTL), identifying a promising candidate, Dixdc1.
Methods :
QTL mapping was performed using GeneNetwork. SNPs and InDels were identified in genes at QTL using the SANGER mouse database. Genes that contained variants were further examined and ranked based on the predicted effect of the variant on the protein, gene expression, and gene function, as previously reported (Kulesh et al., 2017, ARVO Abs.). Retinas from adult Dixdc1-knockout (KO) and Dixdc1-wildtype (WT) mixed litters were immunolabeled for Prox1 to determine the number of AII amacrine cells.
Results :
AII amacrine cell number varied notably across the RI strains, from 57,141 cells to 86,557 cells, a nearly 30,000 cell difference. Using composite interval mapping, two significant QTLs were identified on Chrs 9 and 19, modulating AII amacrine cell number by 11,139 and 13,252 cells, respectively. Candidate gene analysis identified 21 promising candidates at these two loci, including Dixdc1 on Chr 9 because of the presence of a missense SNP and another splicing SNP discriminating the parental genomes. Elimination of Dixdc1 led to a significant increase in the number of AII amacrine cells, by about 12,000 cells, being comparable to the Chr 9 QTL effect.
Conclusions :
Dixdc1 participates in the regulation of AII amacrine cell number. DIXDC1 is a positive regulator of the WNT pathway, known to play a role in cell fate specification. Additionally, many functional isoforms of DIXDC1 have been documented in the brain, raising the possibility that the regulation of its splicing affects fate assignment. An SNP in a splicing factor recognition motif is therefore an attractive candidate sequence variant that may contribute to the differences in cell number across strains.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.